1. Field of the Invention
It is a well known fact that wood, coal, natural gas, petroleum and other organic substances are burned in important amounts in private and commercial combustion plants as well as in thermal power engines. If too much air or oxygen is fed during the combustion process, a loss occurs because the excessive air carries off a substantial amount of heat. If too little air is fed, there is generated, besides soot, also the toxic carbon monoxide and furthermore hydrocarbons, among them some highly cancerogenic species as e.g. the benze-a-pyrene. The ever increasing contamination of our environment, particularly of the air we are breathing, with cancerogenic substances exhibits a very substantial menace to the population. In order to avoid any danger of explosion also occuring during the production of these harmful substances, particularly heating plants and automobiles without catalytic post-combustion have to be operated with a certain amount of excess air. On the other side it is not desirable at all to spend the valuable fuels unnecessarily by conducting a combustion process with a too great amount of excess air.
2. Prior Art
It is well known for a long time that the ratio between fuel and air, called the factor .lambda., plays an important role during the process of a combustion. Particularly the efficiency and the emission of contaminations of combustion engines and heating plants is determined by the factor .lambda.. Exactly, the factor .lambda. is defined by the ratio EQU =n.sub.O.sbsb.2 (really): n.sub.O.sbsb.2 (ideally).
Thereby n.sub.O.sbsb.2 (really) means the really fed amount of air or oxygen and n.sub.O.sbsb.2 (ideally) the amount of air or oxygen which would be required for a complete combustion of the fuel. In the case of automobiles without catalytic post-combustion and in the case of heating plants, the factor .lambda. must usually be in the region of 1.2 to ensure optimal operating conditions. If the factor .lambda. is 1.1, an emission of contaminations and the danger of explosions must be expected; if the factor .lambda. is 1.3, the efficiency is decreased. However the optimal .lambda.-values are different from case to case. Among other, they depend on the type of the thermal engine and of the firing plant, respectively, as well as on the kind of the fuel to be used.
It is further known that the factor .lambda. may be automatically adjusted, e.g. by means of a zirconium oxide oxygen sensor. It was thereby possible to decrease the fuel consumption of a heating plant by 5 to 10% and, simultaneously, to substantially improve the exhaust gas rates. The disadvantage of the zirconium oxide oxygen sensor lies in the fact that its proper function is impaired under the influence of lead and other substances wich might be contained in the exhaust gas, and it can therefore be used only in conjunction with certain selected kinds of fuel.
An electrode exposed to the exhaust gas must be protected by means of porous ceramic material; there is always a danger that the electrode will be contaminated. Furthermore, the diffusion of O.sub.2 through the ceramic material is comparatively slow, particularly at lower temperatures, resulting in the fact that the control or regulation cycle to be realized by means of such an electrode becomes correspondingly slow as well. In addition it should be noted that a switching signal which is independent of the temperature is offered only if the factor .lambda. has the value of 1.0. This means that, in the case of combustion plants and automobiles without a catalytic post-combustion and operating with an optimal .lambda.-factor, any excess oxygen must be removed by adding a certain amount of H.sub.2 by titration, before the exhaust gas is fed to the solid electrolytic sensor. This requires a mechanism which is comparatively complex and thereby expensive which could be installed only in big combustion plants up to now.